In the final surface finishing of a product (workpiece), the surface of the workpiece needs to be finished to be fine and smooth. For example, a dental prosthesis has a complex irregular surface, and it is time consuming to polish the surface thereof. Japanese Unexamined Patent Application Publication No. 11-347945 discloses a device that surface-finishes a workpiece by spraying numerous abrasive grains in an oblique direction onto the surface of the workpiece. More specifically, sliding the abrasive grains on the surface of a workpiece achieves almost the same final finishing effect as polishing it with sandpaper. In such a case, a cluster of abrasive grains having a certain degree of density and some thickness is preferably impacted onto the workpiece surface in an oblique direction. To increase smoothness of the workpiece surface, the cluster of abrasive grains needs to be ejected continuously in a densely aggregated state at a predetermined grain density when the cluster is impacted onto the workpiece surface.
In known abrasive grain jet grinding devices, the abrasive grains ejected from an impeller are slightly scattered. For this reason, the abrasive grains need to be aggregated together again before being impacted onto the workpiece. The shape of current ejection nozzles does not adequately overcome this problem and meet the necessity to eject the abrasive grains continually in a densely aggregate state.
Since spacing between blades equally spaced on an impeller is relatively wide in the device disclosed in the aforementioned Publication No. 11-347945, the abrasive grains from the impeller are ejected not continuously but intermittently. In such a case, the abrasive grains impacting the workpiece are not densely aggregated, so that the abrasive grains are repelled and bounced in a direction instead of impacting in a direction of grinding and polishing the surface of workpiece. Consequently, such a device may not provide sufficient polishing. This is because in the case of a continuous cluster of mutually closely aggregated numerous abrasive grains, the abrasive grains interfere with each other when the cluster impacts onto the workpiece, and flow in a cluster on the workpiece. The degree of gloss of the polished surface of the workpiece becomes fine. If the abrasive grains are not densely aggregated, the abrasive grains, when impacted on the workpiece, are repelled and the direction of bounce is not stabilized. The polishing efficiency is low, and the polished surface is low in the quality of gloss.